The gate insulating films of transistors formed on silicon substrates require high performance characteristics such as low interface level density, and high reliability such as high withstand voltage and high hot carrier tolerance. As a conventional oxide film formation technique to meet those requirements, thermal oxidation at 800° C. or more has been used.
Besides, from a demand for forming transistors on a silicon semiconductor at higher density, for transistor integrated elements formed on a silicon substrate, in concert with the progress of scale-down technique, in place of selective oxidation film (LOCOS) element isolation structures in which bird's beaks extend, element isolation structures such as shallow trench isolation that enables narrow dielectric isolation came to be used.
Besides, for integrated elements such as SOI (Silicon On Insulator) transistors and polysilicon transistors formed on an insulating film, used were element isolation structures in which silicon films are formed into islands by LOCOS isolation with silicon oxide films and mesa isolation by etching silicon off.
For formation of semiconductor elements with ultrahigh integration/ultrahigh speed drive, however, conventional thermal oxidation can not be used. To realize ultrahigh speed elements, a metallic material must be introduced in the semiconductor device. But, if a high temperature process at 550° C. or more is used, the metal can react with the semiconductor to deteriorate the operation performance of the elements. Besides, if such a high temperature process is used, it becomes difficult to form an accurate impurity distribution because of rediffusion of impurities. This makes it hard to form ultrahigh integrated elements. Therefore, oxide film formation at a low temperature of 550° C. or less is indispensable.
So, in recent years, techniques for forming silicon oxide films at low temperatures have been studied. But, characteristics of a silicon oxide film formed at 550° C. or less were never equal to those of a thermal oxide film. The oxidation speed of such conventional low-temperature oxidation is lower than that of thermal oxidation, so electrical characteristics, such as interface level density and current-voltage characteristic, of a silicon oxide film formed were greatly inferior to those of a thermal oxide film.
Besides, in a conventional element isolation structure for transistor integrated elements formed on a silicon substrate, the thickness of a silicon oxide film at a portion near a corner of an element isolation side wall portion is smaller than that on a flat silicon surface portion. Therefore, a problem arose that characteristics such as leakage current and withstand voltage of the oxide film are inferior at the thin portion and the reliability in performance of the elements are deteriorated. Further, since a parasitic transistor element having its thin gate oxide film exists in parallel with a transistor element having its gate oxide film of a normal thickness, this deteriorated the voltage-current characteristic of the transistor.
In order to solve such problems, if the thickness of the silicon oxide film is simply increased to avoid the problem that arises at the thin portion, since the silicon oxide film also serves as a gate oxide film, a problem arises that the drive performance of the MOS transistor deteriorates. So, conventionally, the angle of the side wall portion of the recessed portion in the element isolating region with the silicon surface is set at about 70 degrees or less so that thinning of the silicon oxide film at corners of the side wall portion is relieved. Even in this case, however, about 30% or more thinning occurred, and occurrence of characteristic deterioration such as leakage current and withstand voltage of the oxide film at the thin portion could not completely be prevented. Furthermore, formation of the recessed element isolating region with an obtuse angle brought about problems that the element isolation width increased, the ratio in area of the effective region where elements such as transistors are to be formed decreased, and high density integration could not be intended.
Furthermore, in a conventional element isolation structure for integrated elements such as SOI (Silicon On Insulator) transistors and polysilicon transistors formed on an insulating film, in case of LOCOS element isolation, a parasitic transistor element existed near the interface between the element isolation oxide film below a gate electrode and silicon. This deteriorated electrical characteristics of the transistor, in particular, subthreshold current characteristic and off-leak characteristic. On the other hand, in case of mesa element isolation, a high quality oxide film could not be formed on the element isolation side wall portion where silicon has been etched off. This had a bad influence on characteristics, in particular, off characteristics, of the transistor.
Accordingly, it is an object of the present invention to provide semiconductor devices, and methods and apparatus for forming silicon oxide films, which make it possible to realize silicon oxide films having, even though they were formed by low-temperature plasma oxidation, characteristics and reliability superior to those of silicon thermal oxide films formed at a high temperature of about 1000° C., and to realize high-performance transistor integrated circuits that can reduce the area of their element isolating regions and be suitably applied to SOI transistors and TFTs.